1. FIELD OF THE INVENTION
The present invention relates to a method for improving the quality of seismic data and more particularly to depth migrating data before stacking to improve the quality of seismic data acquired in the presence of complex structures or any laterally changing velocity.
2. RELATED PRIOR ART
Presently, prestack traces are usually aligned assuming three characteristics. First, the common depth point (CDP) is halfway between the source and receiver. Second, all seismic reflectors are in the plane of the section. Third, all traces within an area designated as a common depth point can be corrected to zero offset by using the same velocity function.
There are many methods in the prior art for aligning seismic data. For example, U.S. Pat. Nos. 4,766,574 and 4,742,497 illustrate the alignment of seismic data migrated before stack.
U.S. Pat. No. 4,766,574 titled "Method for Depth Imaging Multicomponent Seismic Data", (Norman D. Whitmore, Jr., et al.), relates to a method of migrating time dependent reflectivity functions prior to stacking to obtain depth images of the earth's subsurface geological structure as well as estimates of shear and compressional wave interval velocities. Measures are obtained of generated seismic wavefields incident on reflecting interfaces or subsurface layer boundaries in the earth's crust. Measures are also obtained of resulting seismic wavefields scattered from these interfaces. The incident and scattered seismic wavefields are employed to produce time-dependent reflectivity functions representative of the reflecting interfaces. By migrating these time-dependent reflectivity functions, depth images of the reflecting interfaces can be obtained. For pairs of multicomponent seismic data, the dyadic set of multicomponent seismic data are partitioned so as to separate the variously coupled incident and reflected wavefields in the recorded multicomponent seismic data. The incident and reflected wavefields are cross-correlated to form time-dependent reflectivity functions. These time-dependent reflectivity functions are then iteratively migrated according to a model of wavefield velocities of propagation to obtain better estimates of the compressional and shear wave interval velocity. The migrated reflectivity functions can then be stacked to produce better depth images of the earth's subsurface geological structures.
U.S. Pat. No. 4,742,497 titled "Method of Processing Seismic Data", (Craig Beasley. et al.), relates to a method for converting a set of areal-distributed seismic traces without dependence on subsurface structure orientation dependent velocities. A three-dimensional display of a volume of the earth without use of dip-dependent or azimuth-dependent migration velocities.
Once data is aligned, various other methods may be used for migration. U.S. Pat. Nos. 4,745,585, 4,479,205 and 4,464,737 along with United States Statutory Registration number H482 illustrate methods of migrating seismic data.
U.S. Pat. No. 4,745,585, "Method of Migrating Seismic Data" (Kenneth L. Larner), relates to seismic data which is passed through a preselected number of migration stages. During each stage, data is migrated a plurality of times, where the migration-velocity function is a minor fraction of the velocity required to fully migrate the data in a single stage. The cascaded migration migrates data having steeply-dipping events with what is alleged to be greater noise reduction than does a single-stage migration.
U.S. Pat. No. 4,479,205 titled "Method of Migrating Seismic Data without Dependency on Velocity", (William C. Gray), relates to a method for migration of velocity spectra wherein migration of common depth point gathers is accomplished without prior determination of the velocity/depth field. By using the method presented, in which the velocity spectra of the data is transposed, fast Fourier transformed and then migrated, migration of the data is accomplished without prior determination of velocity.
U.S. Pat. No. 4,464,737 titled "Method for Migration of Seismic Reflection Waves", (Keh Pann) relates to seismic exploration wherein a seismic source transmits seismic energy into the earth and seismic detectors record the seismic waves returning to the earth's surface from reflecting subsurface interfaces. The downgoing transmitted seismic energy and the upcoming reflected seismic waves are transformed into the frequency domain, downward continued, and summed to produce a migrated seismic depth section.
United States Statutory Invention registration number H482, titled "Seismic Migration Method", (John R. Berryhill, et al.), relates to a seismic data processing method in which seismic traces are subjected to Fourier transformations, the coefficients of the Fourier-Transformed traces are subjected to a recursive KF migration operation and the migrated traces are then inverse Fourier-transformed. Each trace contains a signal resulting from reflection of a seismic wave at a location within the earth. Each trace is associated with at least one point in a two dimensional spatial grid (x,y). When displayed, the processed seismic data represents the position within the earth of whatever caused the reflection. The method may be employed to process stacked seismic traces, each associated with a single point (x,y) in the grid, or may be employed to process unstacked seismic traces, each associated with both a seismic source location (x.sub.s,y.sub.s) and a different seismic receiver location (x.sub.r,y.sub.r) in the grid. In performing the method, the earth is modeled as a stack of M horizontal layers each characterized by a seismic wave velocity. The recursive KF migration step is iterated M-1 times for each trace, where part of the output of each iteration is stored and part is discarded.